Method for evaluating layers of images

ABSTRACT

The invention is directed to a method for evaluating layer images (A, B, C, D, E) which are recorded by microscope from planes of different depths in focusing direction z of an object. Every layer image (A, B, C, D, E) is composed of a plurality of image points (A ij , B ij , C ij , D ij , E ij ); an intensity value is determined for every image point (A ij , B ij , C ij , D ij , E ij ) or for image areas comprising a plurality of these image points (A ij , B ij , C ij , D ij , E ij ); the intensity values for image points (A ij , B ij , C ij , D ij , E ij ) or image areas lying one above the other in z-direction are combined; a parameter characteristic of these image points (A ij , B ij , C ij , D ij , E ij ) or image areas is determined and ordered within a grid corresponding to the grid of the image points (A ij , B ij , C ij , D ij , E ij ). In this way, information about the topography of the object, for example, can be obtained and displayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of International Application No.PCT/EP02/02881, filed Mar. 17, 2002 and German Application No. 101 12947.5, filed Mar. 17, 2001, the complete disclosures of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention is directed to a method for evaluating layer imageswhich are recorded by microscope from planes of different depths infocusing direction z of an object.

[0004] 2. Description of the Related Art

[0005] In scanning microscopy, an object being examined is scanned pointby point under defined conditions of measurement light. In so doing, theintensity of the measurement light is detected for each individualobject point and an equivalent of the intensity value is associated ineach instance with an image point of an image.

[0006] As a rule, images of an object space or images from differentobject depths are generated in this way from a plurality of differentplanes in the focusing direction, which usually corresponds to thez-direction. Information about characteristics of the examined objectcan then be obtained from the measured intensity values. For example,information about the fine surface structure or the layer constructionof an object can be obtained in this way. This is important for theinspection of semiconductor components, particularly wafers, among otherthings.

[0007] Confocal scanning microscopes working in the range of visiblelight that is usable for this purpose or in the near UV range arealready known in general. Image recording is carried out, for example,by means of a Nipkow disk. A confocal scanning microscope of this kindis described in German Patent 195 11 937

[0008] In polychromatic confocal scanning microscopes, the bandwidth ofthe visible light with its different wavelengths is used for recordinglayer images. The light of different wavelengths is imaged onobservation planes located at various depths. In this case, intensityvalues from the different planes can be detected by a measuring process.

[0009] By contrast, it is also possible to record the layer images withmonochromatic confocal scanning microscopes or with laser scanningmicroscopes. For this purpose, the individual planes are successivelybrought into focus and the intensity of the measurement light isdetected in each instance.

OBJECT AND SUMMARY OF THE INVENTION

[0010] Proceeding from this prior art, it is the primary object of theinvention to provide an improved method for evaluating the layer imagesobtained in scanning microscopy by which precise information aboutobject characteristics can be obtained in an efficient manner.

[0011] This object is met by a method of the type mentioned above,wherein every layer image (A, B, C, D, E) is composed of a plurality ofimage points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) arranged in agrid, an intensity value is determined for every image point (A_(ij),B_(ij), C_(ij), D_(ij), E_(ij)) or for image areas comprising aplurality of these image points (A_(ij), B_(ij), C_(ij), D_(ij),E_(ij)), the intensity values for image points (A_(ij), B_(ij), C_(ij),D_(ij), E_(ij)) or image areas lying one above the other in z-directionare combined according to given criteria, a parameter characteristic ofthese image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) or imageareas is determined, and the parameters relating to these image points(A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) or image areas are correlatedwith the elements of a grid corresponding to the grid of the imagepoints (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) in a layer image (A, B,C, D, E).

[0012] By image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) ismeant, for example, the pixels or subpixels of an LC display;consequently, image areas may comprise a plurality of neighboring pixelsor subpixels of a display of this kind. In other words, the image points(A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) are the smallest units on whichimage information can be displayed or by which image information can bedetected, while the above-mentioned image areas extend over a largersurface area than the image points (A_(ij), B_(ij), C_(ij), D_(ij),E_(ij)). The image areas in the different planes lying one above theother in z-direction can be of different sizes, i.e., they can comprisedifferent quantities of image points (A_(ij), B_(ij), C_(ij), D_(ij),E_(ij)) in different planes. The size of the image areas depends, forexample, upon defocusing while determining the measurement values. Forthe sake of simplicity, the invention will be described in the followingonly with reference to the evaluation of individual image points(A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)).

[0013] Using the given criteria, determined characteristics of theobject at the specified location can be determined for every recordedobject point or for the immediate vicinity of the object point. Forexample, information about the geometry of the object surface or aboutthe geometry of a boundary surface can be derived from the intensityvalues. By deliberately condensing or selecting such information, a datafield similar to the grid structure of the layer images can then begenerated, and this data field can be displayed graphically.

[0014] In an advantageous arrangement of the invention, the extremalvalue of the intensity values is determined for the image points lyingone above the other. A quantity characterizing the position inz-direction is determined at the extremal intensity value and iscorrelated with the characteristic parameter. In this way, based on oneof the object points lying one above the other whose position inz-direction is known, the layer image having the maximum intensity atthis point is determined.

[0015] The presence of a boundary layer or surface layer can be deducedfrom the intensity, so that an image is formed of the characteristicparameters. This image represents a depiction of the surface topographyof the object to be examined or the topography of a boundary layer witha determined reflection behavior.

[0016] An approximation curve for the intensity variation is preferablygenerated for the image points lying one above the other in theindividual image planes, which approximation curve has the intensityvalues of these image points as nodal points. A quantity characterizingthe position in z-direction is determined for the extremal value of theapproximation curve within a depth area and is correlated with thecharacteristic parameter. This procedure allows a more accuratedetermination of the position of the intensity maximum which can also belocated between the z-position of two adjacent layer images for anobject point. A particularly high resolution in z-direction is producedin this way.

[0017] In another advantageous arrangement of the invention, theextremal value of the intensity values of the image points lying oneabove the other is correlated with the characteristic parameter withoutreference to the z-position. The characteristic parameters for theindividual object points accordingly represent information about thespatial reflection behavior of the examined object.

[0018] The extremal value of an approximation curve in the verticalobject area which is represented by the layer images and which has theintensity values of the image points lying one above the other as nodalpoints is preferably correlated with the characteristic parameter. Inthis way, the local intensity maximum can be determined in aparticularly accurate manner for the individual object points.

[0019] The mathematical methods used for generating the approximationcurve are known in general and need not be described more fully herein.However, it is essential that the characteristic parameter is obtainedfor all object points on the basis of the same criterion, i.e., based onthe same approximation rule.

[0020] For a particularly highly accurate evaluation it has provenadvantageous to determine the function type of the approximation curveby a calibrating process. In particular, this also takes into accountthe apparatus characteristics of the optical system used for generatingthe layer images. The calibration curve to be taken as a basis can bedetermined empirically or can be calculated by a theoretical route.

[0021] The grid structure of the elements is adapted to the structure ofthe image points of the layer images in order to obtain meaningfulresults which are as accurate as possible. In scanning microscopy, CCDcameras are generally used for generating image information andintensity values. Consequently, it is particularly advantageous when thegrid structure of the elements to which the characteristic parametersfor the individual object points are assigned is formed of rows andcolumns.

[0022] In another advantageous arrangement of the invention, the layerimages are recorded in object planes located equidistant from oneanother one above the other. This has the advantage that it keeps downcomputing time for evaluating the image points lying one above the otherin the individual object planes, particularly when determining theapproximation curves and their maximum.

[0023] Of course, it is also possible for the evaluation to be basedupon layer images originating from object planes with different relativedistances from one another. This can be advantageous particularly whenit is difficult to generate equidistant layer images. In this case,additional distance information must be taken into account in theevaluation.

[0024] In optical systems which are used, for example, to generate layerimages, the resolution depends among other things on the wavelength ofthe measurement light. When the layer images are generated withmeasurement light of different wavelengths, they have varying resolutionin z-direction.

[0025] Therefore, in an advantageous arrangement of the invention, theintensity values of the layer images are related to a monochromaticlight. Accordingly, a uniform resolution over the entire object space tobe examined is achieved in the z-direction as well as in an xy-planeperpendicular to the z-direction. Layer images of this kind can beobtained, for example, with a monochromatic confocal scanning microscopeor also with a laser scanning microscope.

BRIEF DESCRIPTION OF THE DRAWING

[0026] In the accompanying drawing, FIG. 1 is a schematic view of layerimages lying one above the other, each of which has a plurality of imagepoints with which intensity values are associated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The invention is described more fully below with reference to anembodiment example.

[0028] A plurality of layer images of an object space to be examined aregenerated by a confocal scanning microscope for different object depthsin z-direction. The scanning microscope used for this purpose may be,for example, a confocal scanning microscope which is operated withmeasurement light in the UV range. The wavelength range of themeasurement light is very small, so that a plurality of separaterecordings must be made within the framework of a focus series for theindividual layer images in the z-direction. These layer images are shownschematically in FIG. I and are designated by A, B, C, D and E. Thequantity of layer images is not limited to the quantity shown in FIG. 1,but is essentially freely selectable.

[0029] Each of the layer images shown, A, B, C, D, E, has a gridstructure with a plurality of image points which are arranged in rows iand columns j. In FIG. 1, the image points A_(ij), B_(ij), C_(ij),D_(ij), E_(ij) lying one above the other are shown for an object areaextending in z-direction along the depth, which corresponds to the sumof distances d_(AB) to d_(DE).

[0030] An intensity value that was measured during the generation of therespective layer image A, B, C, D, E at a reception device of thescanning microscope is associated with each of these image pointsA_(ij), B_(ij), C_(ij), D_(ij), E_(ij). This reception device is usuallya matrix of a CCD camera.

[0031] Instead of the aforementioned confocal scanning microscopeoperated in the UV range, a confocal scanning microscope operated with amonochromatic measurement light can also be used. In this case, a veryuniform resolution is achieved for all layer images A, B, C, D, E inz-direction. Alternatively, a laser scanning microscope can also beused.

[0032] Further, in all cases in which the layer images A, B, C, D, E arerecorded successively by focusing on different object planes inz-direction, the distance d_(AB), d_(BC), d_(CD) or d_(DE) betweenneighboring layer images is fixed. Alternatively, the distance from apredetermined reference point (not shown in the drawing) to eachindividual layer image A, B, C, D, E or to the associated object planecan be recorded.

[0033] Further, it is conceivable to generate the layer images A, B, C,D, E by means of a broadband polychromatic confocal scanning microscopein which focusing in z-direction is carried out by wavelength selection.This is also possible in an analogous manner when operating the confocalscanning microscope in the visible spectral range of light insofar asthe resulting image is broken down into color values with which depthinformation is correlated.

[0034] The intensity values recorded in the individual layer images A,B, C, D, E for the image point A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)can be evaluated in different ways, as will be described more fully inthe following, in order to obtain information about objectcharacteristics.

[0035] A “best focus image” is generated from the layer images to showthe topography of the objet under examination. The effect whereby aclear intensity peak is adjusted when the scanning microscope is focusedon a boundary surface is utilized for this purpose. This is especiallyclearly pronounced at the object surface. Beyond this, less distinctlypronounced secondary intensity peaks can occur in partially transparentbodies.

[0036] In order to generate the best focus image, the image points ofthe layer images lying one above one another, i.e., the image pointswith the same index, are evaluated according to a predeterminedcriterion while generating a characteristic parameter. In the embodimentexample, this criterion is an approximation curve determined by typewith which the intensity curve is approximated or fitted in the objectdepth range represented by the layer images A, B, C, D and E.

[0037] The intensity values measured at the individual image pointsA_(ij), B_(ij), C_(ij), D_(ij), E_(ij) form the nodal points of theapproximation curve. Further, the distances d_(AB), d_(BC), d_(CD) andd_(DE) between the layer images A, B, C, D, E in z-direction are takeninto account when parameterizing the approximation curve. Insofar asthese distance d_(AB), d_(BC), d_(CD), d_(DE) are identical for alladjacent layer images A, B, C, D, E, this can even be taken into accountin the function rule, so that the approximation curve can beparameterized based on the intensity values alone.

[0038] For the approximation curve, the extremal value of the intensityis determined within the above-mentioned object depth range and theassociated position in z-direction is determined with reference to thisextremal value.

[0039] Therefore, a pair of values consisting of a value for theintensity and a z-quantity is obtained. When the best focus image isgenerated, this z-quantity and an element of a grid structure that isvery similar to the grid structure of the image points A_(ij), B_(ij),C_(ij), D_(ij), E_(ij) in a layer image A, B, C, D, E are assigned tothe characteristic parameter.

[0040] In this way, the characteristic parameters can be determined overall indices and can be collected in a data field. This data field isthen displayed, e.g., visually, as best focus image, which is asynthetic image.

[0041] Due to the similarity of the grid structure to that of the imagepoints A_(ij), B_(ij), C_(ij), D_(ij), E_(ij), an image results whichcontains scaled topographic information. Since the displayed informationis based on object points that are actually measured, a quantitativescaling is achieved in contrast to confocal scanning microscopes inwhich a chromatic observation is carried out.

[0042] The approximation curves mentioned above can also be evaluatedwith respect to the extremal value of the intensity for the individualimage points A_(ij), B_(ij), C_(ij), D_(ij), E_(ij), and, therefore, forthe corresponding object points and can be assembled to form a syntheticimage. In this case, the respective maximum intensity value of theapproximation curve in the object depth range represented by the layerimages A, B, C, D, E is correlated with the characteristic parameter.The synthetic image then gives an isodose distribution of the extremalintensity which can be evaluated further.

[0043] With structured surfaces in which different materials differ withrespect to their refection behavior, the materials and therefore thestructure planes can be displayed in a corresponding manner. For thepurpose of quantification of the synthetic images generated in this way,a calibration is possibly carried out beforehand at a surface withconstant reflectivity, e.g., a mirror.

[0044] In a simplified modification of the method steps mentioned above,the generation of an approximation curve is omitted. Instead, forpurposes of displaying the topography in the form of a best focus image,the z-quantity of the layer image A, B, C, D, E at which the intensitymaximum for the image points A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)lying one above the other is determined is directly correlated with thecharacteristic parameter of an object point.

[0045] On the other hand, for displaying the isointensity surfaces, themaximum intensity value is directly correlated with the characteristicparameter from the image points A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)lying one above the other.

[0046] When the method described above is used to examine a layer systemhaving a plurality of layers of identical constitution and reflectivity,both the best focus image and the isointensity surface depiction areused for obtaining information, e.g., in order to show isodosedistributions on depth structures and, accordingly, to resolve thestructure of the layer system.

[0047] Further, additional information about properties of the objectcan be derived from the approximation curves or evaluation functions,for example, by comparing with reference curves. For example, in atleast partially transparent objects, boundary layers located within theobject can be deduced based on the determination of secondary maxima.When the object to be measured is fundamentally known with respect toits structure, defective locations can be deduced based on intensitydeviations that are determined in this manner.

[0048] The method according to the invention can be carried out inincident light operation as well as transmitted light operation.

[0049] While the foregoing description and drawings represent thepresent invention, it will be obvious to those skilled in the art thatvarious changes may be made therein without departing from the truespirit and scope of the present invention.

[0050] Reference Numbers:

[0051] A, B, C, D, E layer images

[0052] A_(ij), B_(ij), C_(ij), D_(ij), E_(ij) image point

[0053] i rows

[0054] j columns

[0055] d_(AB), d_(BC), d_(CD), d_(DE) distances

1. Method for evaluating layer images (A, B, C, D, E) which are recordedby microscope from planes of different depths in focusing direction z ofan object, wherein every layer image (A, B, C, D, E) is composed of aplurality of image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij))arranged in a grid, an intensity value is determined for every imagepoint (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) or for image areascomprising a plurality of these image points (A_(ij), B_(ij), C_(ij),D_(ij), E_(ij)), the intensity values for image points (A_(ij), B_(ij),C_(ij), D_(ij), E_(ij)) or image areas lying one above the other inz-direction are combined according to given criteria, a parametercharacteristic of these image points (A_(ij), B_(ij), C_(ij), D_(ij),E_(ij)) or image areas is determined, and the parameters relating tothese image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) or imageareas are correlated with the elements of a grid corresponding to thegrid of the image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) in thelayer images (A, B, C, D, E).
 2. Method according to claim 1,characterized in that the extremal value of the intensity values isdetermined for image points (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij))lying one above the other, a quantity characterizing the position inz-direction is then determined at this extremal intensity value and isassigned as a value to the characteristic parameter.
 3. Method accordingto claim 1, characterized in that an approximation curve for anintensity variation is generated for each of the depths in focusingdirection z in which a layer image (A, B, C, D, E) is obtained, whereinthe intensity values of the image points (A_(ij), B_(ij), C_(ij),D_(ij), E_(ij)) or image areas lying one above serve as nodal points,the extremal value of the approximation curve is determined according tothe latter, and a quantity characterizing the position in z-direction isthen determined and is assigned as a value to the characteristicparameter.
 4. Method according to claim 1, characterized in that theextremal value of the intensity values of the image points (A_(ij),B_(ij), C_(ij), D_(ij), E_(ij)) or image areas lying one above the otheris correlated with the characteristic parameter.
 5. Method according toclaim 1, characterized in that the extremal value of an approximationcurve within the vertical object area which is represented by the layerimages (A, B, C, D, E) is correlated with the characteristic parameter,wherein the approximation curve has the intensity values of the imagepoints (A_(ij), B_(ij), C_(ij), D_(ij), E_(ij)) or image areas lying oneabove the other as nodal points.
 6. Method according to claim 3 or 5,characterized in that the function type of the approximation curve isdetermined by a calibrating process.
 7. Method according to one of thepreceding claims, characterized in that the grid structure is composedof rows and columns.
 8. Method according to one of the preceding claims,characterized in that the distances, measured in z-direction, betweenthe planes from which the layer images (A, B, C, D, E) are recorded areequal.
 9. Method according to one of the preceding claims, characterizedin that the intensity values relate to monochromatic light.
 10. Methodaccording to one of the preceding claims, characterized in that theintensity values for the image points (A_(ij), B_(ij), C_(ij), D_(ij),E_(ij)) or image areas are obtained by a confocal scanning microscopewith Nipkow disk or a laser scanning microscope.